This invention relates to a solid-state laser device which uses a laser medium of a slab shape. It is to be noted in the present specification that such a laser medium will be referred to as a slab-shaped laser medium, respectively.
In general, a slab shaped laser medium used in a solid-state laser device of the type described is excited or pumped to generate a laser beam by the use of an optical source, such as a flash lamp, an arc lamp, a light emitting diode (LED), or a laser diode, and a resonator formed by a pair of mirrors. At any rate, it is necessary in such a solid-state laser device to effectively convert energy given by the optical source into beam energy of the laser beam.
Herein, it is mentioned that the energy given by the optical source is converted not only into the beam energy but also into thermal energy in the slab shaped laser medium. The thermal energy brings about a rise in temperature of the slab shaped laser medium and degrades conversion efficiency of the lamp energy. Accordingly, the rise in temperature of the slab shaped laser medium must be avoided so as to achieve high conversion efficiency of the energy given by the optical source. However, it is very difficult to prevent the slab shaped laser medium from the rise in temperature because the slab shaped laser medium is formed by a crystal, such as glass, alexandrite, or the like, which has a comparatively low thermal conductivity.
Under the circumstances, such a solid-state laser device has been used as a laser device of a pulse oscillation type. In this type of the laser device, an optical source is turned on and off at a predetermined period to excite a slab shape laser medium in a time division fashion and to consequently cause the slab shape laser medium to generate a pulsed laser beam. With this structure, the slab shape laser medium is periodically kept at a quiescent state and is cooled during the quiescent state of the optical source. However, it takes a long time to favorably cool the slab shaped laser medium because of a low thermal conductivity of the slab shaped laser medium. In this connection, the quiescent state should inevitably last for a long time to conveniently avoid a rise in temperature of the slab shaped laser medium. Accordingly, it is difficult to raise a repetition frequency of the pulsed laser beam generated from the slab shaped laser medium.
In an article contributed by Robert L. Byer et al to "Optical Letters" vol. 11(10), pp. 617-619 and entitled "40-W Average Power, 30-Hz Moving-Slab Nd:glass laser," a solid-state laser device has been proposed so as to raise a repetition frequency of a pulsed laser beam, as will later be described with reference to one figure of the accompanying drawing. More particularly, the solid-state laser device comprises a slab shaped laser medium having a pair of principal surfaces and a pair of optical sources which are confronted with each other at a predetermined position with a spacing left between. A pair of metallic plates is located between the respective principal surfaces and the optical sources. Under the circumstances, the slab shaped laser medium is moved towards the optical sources. As a result, the slab shaped laser medium is partially and successively illuminated at the predetermined position by the optical sources. Thus, the principal surfaces of the slab shaped laser medium are entirely scanned by the optical sources at the predetermined position, forming a local laser or excited zone through which a laser beam is generated in the form of the pulsed laser beam. In this event, the local laser zone is successively moved in a movement direction at speed determined by movement speed of the slab shaped laser medium. With this structure, it is possible to avoid a rise in temperature of the slab shaped laser medium by selecting the movement speed of the slab shaped laser medium.
However, the slab shaped laser medium should be moved in the device proposed by Byer et al by a mechanism which should have a high reliability. This shows that operation is dependent on the reliability of the mechanism. Moreover, average output power of the laser beam is restricted by the optical sources.
In addition, unevenness of a temperature distribution takes place in the slab shaped laser medium due to shift or movement of the local laser zone. Such a shift of the local laser zone brings about occurrence of a heat gradient in the slab shaped laser medium because a high temperature portion and a low temperature one appear in the slab shaped laser medium. Such a heat gradient gives rise to nonuniformity of the laser beam and, as a result, to degradation of a beam quality of the laser beam.